1. Field of the Invention
The present invention generally relates to a tandem-type image forming apparatus and particularly relates to an intermediate transfer type image forming apparatus.
2. Description of the Related Art
Tandem-type color image forming apparatuses, which have a plurality of photoconductors, are more efficient because they can output more pages per unit time. However, the tandem-type color image forming apparatuses have a disadvantage that, when superimposing all the monochrome toner images formed on the photoconductors on one another to form a full-color image, it is difficult to align writing positions of the toner images. If the writing positions are not aligned correctly, the image quality degrades.
Two types of tandem-type color image forming apparatuses are known: a direct transfer type and an intermediate transfer type. In the direct transfer type, toner images are directly transferred from the photoconductors onto a final transfer material. In the intermediate transfer type, toner images are first transferred from the photoconductors onto an intermediate transfer member, and then onto the final transfer material. The final transfer material is, for example, a paper.
It is easier to correctly align the writing positions of the toner images in the intermediate transfer type than in the direct transfer type. This is because, the toner images are first transferred onto the intermediate transfer member of a certain material in the intermediate transfer type, whereas the toner images are transferred onto various transfer materials in the direct transfer type.
Because the tandem-type color image forming apparatus includes a plurality of photoconductors, the direct transfer type requires a longer transfer material conveying belt and the intermediate transfer type requires a longer intermediate transfer belt than when there is only one photoconductor. Moreover, the belts are extended around a plurality of rollers, which makes the structure complex.
Japanese Patent Application Laid Open No. H8-305184 discloses a direct transfer type image forming apparatus in which a driving roller and a subordinate roller are made to serve as transfer charging rollers whereby the number of rollers can be reduced and a shorter transfer material conveying belt can be used. However, the issue of difficulty in aligning the toner images remains unsolved.
Japanese Patent Application Laid Open No. H7-43976 discloses an image forming apparatus including four fan-shaped image forming units for black, yellow, magenta, and cyan, arranged in a circular ring, which transfer toner images onto an intermediate transfer belt at a single primary transfer position. Moreover, a subordinate roller is caused to function as a transfer charging roller. As a result, the number of rollers can be reduced, and a shorter transfer material conveying belt can be used. Moreover, because there is only one primary transfer position, costs are reduced and the apparatus can be made compact. However, there is a disadvantage that the output capacity of color images per unit time is small.